Process for purifying terephthalic acid



United States Patent U.S. Cl. 260-525 7 Claims ABSTRACT OF THEDISCLOSURE A processs for purifying terephthalic acid in a mixtureobtained as a result of the oxidation of para xylene with oxygen ornitric acid which involves treating such mixtures with a mixture of alower alcohol and a carbonyl-containing compound or a diether.

This is a continuation-in-part application of our application Ser. No.376,586, filed June 19, 1964, and now Patent No. 3,370,088.

This invention relates to a process for purifying an aromatic acidselected from the group consisting of benzophenone 4,4'-dicarboxylicacid and terephthalic acid, and, more particularly, relates to a processfor separating benzophenone 2,4'-dicarboxylic acid from a mixturecontaining the same and benzophenone 4,4-dicarboxylic acid as well as toa process for separating terephthalic acid from a mixture containing thesame and one or more impurities admixed therewith, such as partcarboxybenzaldehyde, para toluic acid, nitrogen-containing impurities,etc.

When ditolylethane is prepared in accordance with known procedures themixture obtained will contain from about 50 to about 90 percent byWeight of the symmetrical isomer, l,l-di (p-tolyl) ethane and from about10 to about 50 percent by weight of the unsymmetrical isomerl-(o-tolyl), l-(p-tolyl) ethane. Even when ditolylethane is prepared bycondensing one mole of acetaldehyde with two mols of toluene in themanner set forth in US. Patent No. 3,002,034 of Schultz, a mixturecontaining about 80 to about 90 percent by weight of 1,1-di (p-tolyl)ethane and about 10 to about 20 percent by weight of l-(otolyl),l-(p-tolyl) ethane is obtained. It is practically impossibleeconomically to separate these isomers from each other, since theirboiling points are so close to each other.

Ditolylethane can be converted to the corresponding acid by subjectingthe same to oxidation conditions in any suitable manner, for example, byfollowing the conditions set forth in US. Patent No. 3,075,007 ofMcCracken et al. Unfortunately, as a result of such oxidation the twodiotolylethane isomers defined above, 1,1- di (ptolyl) ethane andl-(o-tolyl), l-(p-tolyl) ethane, are converted to the acids,benzophenone 4,4'-diacarboxylic acid and benzophenone, 2,4-dicarboxylicacid, respectively.

For many commercial purposes a mixture of the two diacid isomers definedabove is not desired. For example, benzophenone 4,4'-dicarboxylic acidcan be employed as a starting material for the preparation of linearpolyester fibers. The presence of the unsymmetrical isomer, benzophenone2,4'-dicarboxylic acid, would be detrimental in the preparation of suchlinear polymers and, accordingly, must be separated from admixture withbenzophenone 4,4-dicarboxylic acid.

When para xylene is oxidized with a gas containing molecular oxygen,-as, for example, in US. Patent No.

2,853,514 to Brill, or when para xylene is oxidized with nitric acid,as, for example, in US. Patent No. 2,636,899 to Burrows et al., thedesired terephthalic acid obtained is admixed with a significant amountof impurities, such as para carboxybenzaldehyde and para toluic acid. Inthe case of the latter process wherein nitric acid is employed as theoxidant significant amounts of nitrogen-containing impurities are alsopresent in admixture with the terephthalic acid.

For many commercial purposes a mixture of terephthalic acid and thedefined impurities is not desired. For example, as with benzophenone4,4'-dicarboxylic acid, terephthalic acid can be employed as a startingmaterial for the preparation of linear polyester fibers. The presence ofthe defined impurities would be detrimental in the preparation of suchlinear polymers and, accordingly, must be separated from admixture withterephthalic acid.

We have found that benzophenone 4,4'-dicarboxylic acid and terephthalicacid of improved purity can be obtained from the above mixtures by aprocedure which involves treating such mixtures with a mixturecontaining a selected alcohol and a selected carbonyl-containingcompound.

The alcohol that can be employed for the purification procedure definedherein can be defined by the following general formula:

R'OH

wherein R can be an alkyl group having from one to five carbon atoms,such as methyl, ethyl, propyl, butyl and amyl. Examples of such alcoholsare methanol, ethanol, n-propanol, isopropanol, n-butanol, n-pentanol,etc. Methanol is the preferred alcohol.

The carbonyl-containing compound that can be employed herein with thealcohol defined above can be defined by the following general formula:

wherein R" and R', the same or different, are selected from the groupconsisting of hydrogen and alkyl groups having from one to four carbonatoms, such as methyl, ethyl, propyl and butyl. Examples of suchcarbonyl-containing compounds are formaldehyde, acetaldehyde,npropionaldehyde, n-butyraldehyde, acetone, diethylke'tone,methylethylketone, diisopropylketone, methylpropylketone,ethylpropylketone, diisobutylketone, etc. Acetone is the preferredcarbonyl-containing compound.

As pointed out above, we can employ a mixture containing the definedalcohol and the defined carbonyl-containing compound to selectivelyremove the defined impurities from a mixture containing the same andbenzophenone 4,4'-dicarboxylic acid or terephthalic acid. We have noted,however, that during the purification procedure defined herein and underthe conditions thereof the mixture of acids being purified acts as acatalytic medium to promote the reaction of the defined alcohol with thedefined carbonyl-containing compound to produce a diether. We have alsofound that such diethers are also effective to selectively remove thedefined impurities from a mixture containing the same and benzophenone4,4'-dicarboxylic acid or terephthalic acid.

The reaction which we believe takes place between the defined alcoholand the defined carbonyl-containing compound under the catalytic effectof the mixture of acids being treated can be defined as follows:

Since the reaction defined above is reversible the mixture employed willbe one containing alcohol, carbonyl-containing compound, diether andwater.

We have also found that instead of employing a mixture of the definedalcohol and the defined carbonyl-containing compound, the defineddiether alone that would be produced as a result of the reaction definedabove can be employed in the purification procedure defined herein.Examples of such diethers that can be employed herein aredimethoxymethane, diethoxymethane, 1,1-dimethoxyethane,1,1-diethoxyethane, 1,1-dipropoxybutane, 1,1-di (tert. butoxy) methane,2,2-dimethoxypropane, 2,2-diethoxybutane, 2,2-dipropoxy, 4-methylhexane,3,3-dipentoxypentane, etc. We prefer to employ 2,2-dimethoxypropane.

We have also found that while the alcohol defined above or thecarbonyl-containing compound defined above alone will remove some of thedefined impurities from the mixture to be purified, that an equivalentamount of a mixture of the alcohol and the carbonyl-containing compoundwill remove a larger amount of the defined impurities from said mixturethan the alcohol or carbonylcontaining compound alone. In addition whena mixture of benzophenone 2,4'-dicarboxylic acid and benzophenone4,4-dicarboxylic acid that has been obtained as a result of the nitricacid oxidation of a mixture of the defined symmetrical and unsymmetricalisomers of ditolylethane, or a mixture of terephthalic acid, admixedwith the defined impurities, that has been obtained using molecularoxygen or nitric acid as oxidant, is treated with a mixture of alcoholand carbonyl-containing compound, the benzophenone 4,4-dicarboxylic acidor terephthalic acid recovered has an improved color over that of thecharge mixture or even over the product obtained when either the alcoholor the carbonyl-containing compound alone is employed for the definedpurification. When the defined diether alone is employed the color ofthe recovered acid is even improved over that obtained when the mixtureof alcohol and carbonyl-containing compound is employed. The colorbodies so removed are not believed to be either of the two benzophenonecarboxylic acids defined herein but organic compounds containingnitrogen. In the case wherein terephthalic acid is purified herein thecolor bodies are believed to be nitrogen-containing impurities if nitricacid oxidation was employed. In the case wherein terephthalic acid wasobtained by air oxidation color bodies appear to be polymeric in nature.

The treatment defined herein involves merely bringing the mixturescontaining the desired dicarboxylic acids in contacts with the mixtureof alcohol and carbonyl-containing compound or the defined diether. Thealcohol and carbonyl-containing compound employed can be present in thetreating mixture over a wide range, for example, in a molar ratio ofabout :1 to about 1:10, but since, as noted, it is believed the alcoholand the carbonyl-containing compound react herein to form a diether, weprefer to employ the same in approximately stoichiometric amounts, thatis, about two mols of alcohol to about one mol of thecarbonyl-containing compound. The amounts of alcohol andcarbonyl-containing compound or of diether that can be employed hereincan vary over a wide range. Thus, the mixture of alcohol andcarbonyl-containing compound or of diether employed can be at leasttwice the weight of the mixtures of dicarboxylic acids being treated,but generally will be about five to about 10 times the weight thereof.

The reaction conditions required are not critical. Thus, the temperatureemployed can be from about 40 to about 300 0., preferably from about 100to about 200 C. Pressure is not critical and can therefore be from aboutatmospheric to about 1000 pounds per square inch gauge, or even higher.Preferably the pressure is maintained at about the autogeneous pressureof the reaction mixture which can be, for example, from aboutatmospheric to about 500 pounds per square inch gauge. The duration ofthe treatment is similarly not critical, and, for example, can be fromabout one minute to about 10 hours, preferably from about 30 minutes toabout one hour.

As a result of the treatment defined herein we believe substantially allof the defined impurities are dissolved in the treating medium andperhaps a small amount of benzophenone 4,4-dicarboxylic acid orterephthalic acid is also dissolved therein. The bulk of the latteracids, however, are insoluble in the treating medium. At the end of thetreatment the solid benzophenone 4,4-dicarboxylic acid or terephthalicacid is separated from the treating medium containing the dissolvedimpurities in any convenient fashion, for example, by filtration ordecantation. The solid benzophenone 4,4'-dicarb0xylic acid orterephthalic acid left behind can be washed, if desired, for example,using methanol or acetone.

The procedure of this invention can further be understood by referenceto the following. The benzophenone dicarboxylic acid mixture that wassubjected to treatment in Examples I, II, III and IV was obtained asfollows. There was pumped into an autoclave-522.5 grams of ditolylethaneand 1500 grams of water, and the mixture was then heated to to C. Over aperiod of 1.5 hours and while maintaining the contents of the autoclaveat the defined temperature there was pumped therein 2075 grams of nitricacid having a concentration of 70 percent. The contents of the autoclavewere .then raised to to C. over a period of 15 minutes and maintained atthe latter temperatures for a period of. two hours while the pressure inthe autoclave was held I at 300 pounds per square inch gauge. At the endof this period, the product was filtered and dried at 150 C. There wasobtained 605 grams of the mixture of benzophenone dicarboxylic acidsemployed herein. Examples I and II below show the results obtained whenmethanol and acetone, respectively, are employed to separatebenzophenone 2,4'-dicarboxylic acid from a mixture contain ing the sameand benzophenone 4,4-dicarboxylic acid.

EXAMPLE I Into a one-liter stirred stainless steel autoclave there wasplaced a slurry of 100 grams of a mixture of crude highly yellow coloredbenzophenone dicarboxylic acids analyzed to consist essentially of 85grams of benzophenone 4,4- dicarboxylic acid and 15 grams ofbenzophenone 2,4'- dicarboxylic acid and 700 milliliters of methanol.The mixture was heated over a period of one hour at a temperature of 140C. and a pressure of 140 pounds per square inch gauge. The mixture wascooled to atmospheric temperature and the pressure reduced toatmospheric pressure and then filtered to yield, after drying at 105 C.in a vacuum oven at a vacuum of 20 inches of mercury for two hours, 81.0grams of a solid product.

which was analyzed by means of infrared to consist essentially of 76.65grams of benzophenone 4,4'-dicar-.

boxylic acid and 4.35 grams of benzophenone 2,4'-dicarboxylic acid.There was a slight improvement in the After filtration, there Wasobtained 86.0 grams of a mixture which analysis showed contained 82.1grams of benzophenone 4,4'-dicarboxylic acid and 3.9 grams ofbenzophenone 2,4'-dicarboxy1ic acid. There was a slight improvement inthe color of the product from the original mixture. 1

That a synergistic effect is obtained when a mixture. of the definedalcohol and the defined carbonyl-containing.

compound is employed rather than either one alone is shown by thefollowing.

EXAMPLE III The run of Example I was repeated except that a mixture of560 milliliters of methanol and 140 milliliters of acetone was employedin place of methanol. The liquid in the reactor was analyzed by gaschromotography to contain 30.0 grams of 2,2-dimethoxypropane. Afterfiltration, there was obtained 82.20 grams of a mixture which analysisshowed contained 80.25 grams of hemephenone 4,4'-dicarboxylic acid and1.95 grams of benzophenone 2,4dicarboxylic acid. The color of theproduct was improved somewhat and was only slightly yellow.

Trat a diether is more effective for the defined purpose than a mixtureof alcohol and a carbonyl-containing compound can be seen from thefollowing.

EXAMPLE IV The run of Example I was repeated except that 50 grams of thesame crude benzophenone dicarboxylic acid mixture was subjected totreatment and 350 milliliters of 2,2-dimethoxypropane was employed inplace of methanol. After filtration, there was obtained 40.6 grams of amixture which analysis showed contained 40.3 grams of benzophenone4,4'-dicarboxylic acid and 0.3 gram of benzophenone 2,4'-dicarboxylicacid. The product obtained was near white. In this run, as well as thepreceding runs, the disappearance of some of the benzophenone4,4-dicarboxylic acid can be attributed to some apparent solubility inthe treating medium as well as to handling losses.

In Examples V, VI and VII which follow, the terephthalic acid mixturethat was subjected to treatment therein was obtained as follows. Amixture of 35.4 grams of para xylene, 315 grams of glacial acetic acid,grams of cyclohexane, and 8.0 grams of cobaltous acetate tetrahydratewas reacted with oxygen at a temperature of 90 C. for a period of 34hours. The reaction product was filtered to recover 42.7 grams of solidcrude terephthalic acid, having a neutral equivalent of 90.5. From'thefiltrate 5.7 grams of para toluic acid were obtained along with thecatalyst after evaporation of the acetic acid. Example V and VI belowshow the results obtained when methanol and acetone, respectively, areemployed to separate the defined impurities from a mixture containingthe same and terephthalic acid.

EXAMPLE V Into a 300 milliliter stined stainless steel autoclave aslurry of 10.0 grams of crude terephthalic acid, having a neutralequivalent of 90.5 (theoretical value for pure terephthalic'acid is83.05) and 150 milliliters of methanol were charged. The mixture washeated to a temperature of 145 C. for a period of one hour at aresulting pressure of 165 pounds per square inch gauge. After cooling toroom temperature the mixture was then filtered. The resulting solid wasdried in a vacuum oven at 150 C. and a vacuum of 20 inches of mercury.7.8 grams of purified terephthalic acid, having a neutral equivalent of84.1, was obtained in this manner. The product, however, characterizedby its high neutral equivalent, is not sufficiently pure for polymerapplications. Evaporation of the filtrate, followed by drying theresulting solid in the vacuum oven under the above conditions, produced2.2 grams of solid having a neutral equivalent of 125.4. This materialwas found by infrared and gas chromatographic analysis to consistessentially of para toluic acid (theoretical neutral equivalent 136.14)containing 0.43 gram of terephthalic acid.

EXAMPLE VI The run of Example V was repeated except that 150 millilitersof acetone was employed in place of methanol. After filtration 8.1 gramsof purified terephthalic acid having a neutral equivalent of 86.4 wasobtained. As evidenced by its high neutral equivalent the terephthalicacid after treatment with acetone again is too impure for polymerapplications. The acetone-soluble portion, after evaporation and drying,amounted to 2.0 grams having a neutral equivalent of 192. Analysis byinfrared spectroscopy and gas chromatography shows this fraction to be amixture of para toluic acid, 0.11 gram of terephthalic acid and apolymeric oil, the latter resulting from aldol type condensations ofacetone.

That a synergistic effect is obtained when a mixture of the definedalcohol and the defined carbonyl-containing compound is employed ratherthan either one alone is further shown by the following.

EXAMPLE VII The run of Example V was again repeated except that amixture of 65 milliliters of acetone and 81 milliliters of methanol wasemployed in place of methanol alone. The liquid in the reaction mixturewas analyzed by gas chromatography and found to contain 7.2 grams of2,2-dimethoxypropane. After filtration 8.5 grams of pure terephthalicacid having a neutral equivalent of 83.11 was obtained, which issuitable for polymer applications. It is therefore evident that thecombination of acetone and methanol is more effective in achieving thedesired purification and that this cannot be attained with either of thetwo solvents alone. The acetone-soluble portion, after evaporation anddrying, amounted to 1.5 grams having a neutral equivalent of 130.2. Itwas .analyzed to be a mixture of para toluic acid containing 0.24 gramof terephthalic acid.

EXAMPLE VIII That the defined diether can be obtained herein only whenthe reaction of the alcohol and the carbonyl-containing compounds iscatalyzed by the presence of the benzophenone dicarboxylic acids or theterephthalic acid mixture is apparent from the following. Into aone-liter stirred stainless steel autoclave there was placed 560milliliters of methanol and milliliters of acetone, and the mixture washeated at a temperature of 140 C. and at a pressure of 140 pounds persquare inch gauge over a period of one hour. After cooling to roomtemperature and atmospheric pressure the mixture was analyzed by gaschromatography, but no dimethoxypropane was found.

Obviously, many modifications and variations of the invention ashereinabove set forth can be made without departing from the spirit andscope thereof.

We claim:

1. A process for separating terephthalic acid from a mixture containingthe same, which mixture has been obtained as a result of the oxidationof para xylene with an oxidant selected from the group consisting ofmolecular oxygen and nitric acid, which comprises treating said mixturewith a diether defined by the following formula:

wherein R is an alkyl group having from one to five carbon atoms and R"and R' are selected from the group consisting of hydrogen and alkylgroups having from one to four carbon atoms and thereafter removingterephthalic acid from the resulting mixture.

2. A process for purifying terephthalic acid in a mixture obtained as aresult of the oxidation of para xylene with an oxidant selected from thegroup consisting of molecular oxygen and nitric acid which comprisestreating said mixture with 2,2-dimethoxypropane.

3. A process for purifying terephthalic acid in a mixture obtained as aresult of the oxidation of para xylene with an oxidant selected from thegroup consisting of molecular oxygen and nitric acid which comprisestreating said mixture with 2,2-dimethoxypropane at a temperature ofabout 40 to about 300 C.

4. A process for purifying terephthalic acid in a mixture obtained as aresult of the oxidation of para xylene with an oxidant selected from thegroup consisting of 7 molecular oxygen and nitric acid which comprisestreating said mixture with 2,2-dimethoxypropane at a temperature ofabout 100 to about 200 C.

5. A process for purifying terephthalic acid in a mixture obtained as aresult of the oxidation of para xylene with an oxidant selected from thegroup consisting of molecular oxygen and nitric acid which comprisestreat ing said mixture with a mixture consisting essentially of methanoland acetone.

6. A process for purifying terephthalic acid in a mixture obtained as aresult of the oxidation of para xylene with an oxidant selected from thegroup consisting of molecular oxygen and nitric acid which comprisestreating said mixture with .a mixture consisting essentially of 8methanol and acetone at a temperature of about 40 to about 300 C.

7. A process for purifying terephthalic acid in a mixture obtained as aresult of the oxidation of para xylene with an oxidant selected from thegroup consisting of molecular oxygen and nitric acid which comprisestreating said mixture with a mixture consisting essentially of methanoland acetone at a temperature of about 100 to about 200 C.

References Cited UNITED STATES PATENTS 3,370,088 2/1968 Lese et a1.2605l7 BERNARD HELFIN, Primary Examiner.

